The invention relates to a planetary type transmission with a high transmission ratio.
Such high ratio transmissions are needed for converting rotary motion of a high speed motor shaft, for example, for low speed, high torque applications.
In order to achieve extremely high transmission ratios it is generally necessary to use several transmission stages. This can also be achieved however by a differential arrangement of a planetary transmission with planetary gears having each two sections of different diameters, that is, with a slightly different number of teeth, wherein the sun gear of the planetary transmission and a stationary gear ring each is in engagement with one diameter section and a rotatable gear ring which is connected to the transmission output shaft is in engagement with the other diameter section of the planetary gears. The smaller the difference between the two diameter sections the larger is the transmission ratio, the largest ratio being present if the difference between the two diameter sections is only one tooth. It is noted however that such transmissions develop such large torques that they require a shear pin or a slipping clutch to protect the transmission from overloads in extreme overload situations, for example, if the load locks up.
It is the object of the present invention to provide such a differential type high ratio planetary type transmission which is inherently protected from overloads, which permits to provide for any design ratio and which is relatively inexpensive to manufacture.